Variability: hereditary and non-hereditary. Variability, its types and biological significance

In Darwin's evolutionary theory, the prerequisite for evolution is hereditary variability, and driving forces evolution - the struggle for existence and natural selection. When creating the evolutionary theory, Ch. Darwin repeatedly refers to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believed that the causes of variability are the impact on organisms of factors external environment(direct and indirect), as well as the nature of the organisms themselves (since each of them reacts specifically to the influence of the external environment). Variability serves as the basis for the formation of new features in the structure and functions of organisms, and heredity reinforces these features. Darwin, analyzing the forms of variability, singled out three of them: definite, indefinite and correlative.

A certain, or group, variability is a variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability are an increase in body weight in animal individuals with good feeding, a change in hairline under the influence of climate, etc. A certain variability is massive, covers the entire generation and is expressed in each individual in a similar way. It is not hereditary, that is, in the descendants of the modified group, under other conditions, the traits acquired by the parents are not inherited.

Indefinite, or individual, variability manifests itself specifically in each individual, i.e. unique, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is indefinite, i.e., a trait under the same conditions can change in different directions. For example, in one variety of plants, specimens appear with different colors of flowers, different intensity of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Uncertain variability has hereditary character, i.e., stably transmitted to offspring. This is its importance for evolution.

With correlative, or correlative, variability, a change in any one organ causes changes in other organs. For example, dogs with a poorly developed coat usually have underdeveloped teeth, pigeons with feathered legs have webbing between the fingers, pigeons with a long beak usually long legs, white cats with blue eyes are usually deaf, etc. From the factors of correlative variability, Darwin draws an important conclusion: a person, selecting any structural feature, will almost “probably unintentionally change other parts of the body on the basis of the mysterious laws of correlation.”

Having determined the forms of variability, Darwin came to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors in the evolution of cultural forms are hereditary variability and human selection (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.

Forms natural selection

Selection proceeds continuously over an endless series of successive generations and preserves mainly those forms that are more suitable for given conditions. Natural selection and elimination of some individuals of a species are inextricably linked and are a necessary condition for the evolution of species in nature.

The scheme of action of natural selection in the species system according to Darwin is as follows:

1) Variability is inherent in any group of animals and plants, and organisms differ from each other in many respects;

2) The number of organisms of each species that are born into the world exceeds the number of those that can find food and survive. However, since the abundance of each species is constant under natural conditions, it should be assumed that most of the offspring perish. If all the descendants of any species survived and multiplied, they would very soon supplant all other species on the globe;

3) Since more individuals are born than can survive, there is a struggle for existence, competition for food and habitat. This may be an active life-and-death struggle, or less obvious, but no less effective competition, as, for example, for plants during a period of drought or cold;

4) Among the many changes observed in living beings, some make it easier to survive in the struggle for existence, while others lead to the fact that their owners die. The concept of "survival of the fittest" is the core of the theory of natural selection;

5) Surviving individuals give rise to the next generation, and thus "successful" changes are transmitted through next generations. As a result, each next generation is more adapted to the environment; as the environment changes, further adaptations occur. If natural selection has been operating for many years, then the last offspring may turn out to be so dissimilar to their ancestors that it would be advisable to single them out as an independent species.

It may also happen that some members of a given group of individuals will acquire some changes and be adapted to the environment in one way, while other members of it, having a different set of changes, will be adapted in a different way; in this way from one ancestral species, subject to isolation similar groups There may be two or more types.

driving selection

Natural selection always leads to an increase in the average fitness of populations. Changes in external conditions can lead to changes in the fitness of individual genotypes. In response to these changes, natural selection, using a huge stock of genetic diversity for many different traits, leads to significant shifts in the genetic structure of the population. If the external environment is constantly changing in a certain direction, then natural selection changes the genetic structure of the population in such a way that its fitness in these changing conditions remains maximum. In this case, the frequencies of individual alleles in the population change. The average values ​​of adaptive traits in populations also change. In a number of generations, their gradual shift in a certain direction can be traced. This form of selection is called driving selection.

A classic example of motive selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of the bark of trees covered with lichens, on which it spends daylight hours. Obviously, such a protective coloration was formed over many generations of previous evolution. However, with the beginning of the industrial revolution in England, this device began to lose its importance. Atmospheric pollution has led to the mass death of lichens and the darkening of tree trunks. Light butterflies on a dark background became easily visible to birds. Beginning with mid-nineteenth century, mutant dark (melanistic) forms of butterflies began to appear in the populations of the birch moth. Their frequency increased rapidly. TO late XIX century, some urban populations of the moth were almost entirely composed of dark forms, while light forms still predominated in rural populations. This phenomenon has been called industrial melanism. Scientists have found that in polluted areas, birds are more likely to eat light forms, and in clean areas - dark ones. The imposition of restrictions on atmospheric pollution in the 1950s caused natural selection to change direction again, and the frequency of dark forms in urban populations began to decline. They are almost as rare today as they were before the Industrial Revolution.

Driving selection brings the genetic composition of populations in line with changes in the external environment so that the average fitness of populations is maximum. On the island of Trinidad, guppy fish live in different water bodies. Many of those that live in the lower reaches of the rivers and in the ponds perish in the teeth of predatory fish. In the upper reaches, life for guppies is much calmer - there are few predators. These differences in environmental conditions led to the fact that the "top" and "grassroots" guppies evolved in different directions. The "grassroots", who are under the constant threat of extermination, begin to multiply in more early age and produce many very small fry. The chance of survival of each of them is very small, but there are a lot of them and some of them have time to multiply. "Horse" reach puberty later, their fertility is lower, but the offspring are larger. When the researchers transferred the "grassroots" guppies to uninhabited reservoirs in the upper reaches of the rivers, they observed a gradual change in the type of development of the fish. 11 years after the move, they became much larger, entered breeding later and produced fewer but larger offspring.

The rate of change in the frequencies of alleles in a population and the average values ​​of traits under the action of selection depends not only on the intensity of selection, but also on the genetic structure of the traits that are being selected. Selection against recessive mutations is much less effective than against dominant ones. In the heterozygote, the recessive allele does not appear in the phenotype and therefore eludes selection. Using the Hardy-Weinberg equation, one can estimate the rate of change in the frequency of a recessive allele in a population depending on the intensity of selection and the initial frequency ratio. The lower the allele frequency, the slower its elimination occurs. In order to reduce the frequency of recessive lethality from 0.1 to 0.05, only 10 generations are needed; 100 generations - to reduce it from 0.01 to 0.005 and 1000 generations - from 0.001 to 0.0005.

The driving form of natural selection plays a decisive role in the adaptation of living organisms to external conditions that change over time. It also ensures the wide distribution of life, its penetration into all possible ecological niches. It is a mistake to think, however, that under stable conditions of existence, natural selection ceases. Under such conditions, it continues to act in the form of stabilizing selection.

Stabilizing selection

Stabilizing selection preserves the state of the population, which ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value in terms of adaptive characteristics are removed.

Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them. As a result, individuals with average fecundity turn out to be the most adapted.

Selection in favor of averages has been found for a variety of traits. In mammals, very low and very high birth weight newborns are more likely to die at birth or in the first weeks of life than middle weight newborns. Accounting for the size of the wings of birds that died after the storm showed that most of them had too small or too large wings. And in this case, the average individuals turned out to be the most adapted.

What is the reason for the constant appearance of poorly adapted forms in constant conditions of existence? Why is natural selection unable to once and for all clear a population of unwanted evasive forms? The reason is not only and not so much in the constant emergence of more and more new mutations. The reason is that heterozygous genotypes are often the fittest. When crossing, they constantly give splitting and homozygous descendants with reduced fitness appear in their offspring. This phenomenon is called balanced polymorphism.

sexual selection

In males of many species, pronounced secondary sexual characteristics are found that at first glance seem maladaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features make life difficult for their carriers, making them easily visible to predators. It would seem that these signs do not give any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their origin and spread?

It is known that the survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. C. Darwin called this phenomenon sexual selection. He first mentioned this form of selection in The Origin of Species and later analyzed it in detail in The Descent of Man and Sexual Selection. He believed that "this form of selection is determined not by the struggle for existence in the relationship of organic beings among themselves or with external conditions, but by the rivalry between individuals of the same sex, usually males, for the possession of individuals of the other sex."

Sexual selection is natural selection for success in reproduction. Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition for females arises between males. This competition can be direct, and manifest itself in the form of a struggle for territories or tournament fights. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition is shown in displaying their flamboyant appearance or complex courtship behavior. Females choose those males that they like the most. As a rule, these are the brightest males. But why do females like bright males?

The fitness of the female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, there is a positive Feedback, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability. Imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases, because females choose males not with a certain tail size, but with a larger than average size. In the end, the tail reaches such a length that its harm to the viability of the male is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all that amazing variety of forms, colors and instincts that we observe in the world of wildlife. .



Heredity- This the most important feature living organisms, which consists in the ability to transfer the properties and functions of parents to their descendants. This transmission is carried out with the help of genes.

A gene is a unit of storage, transmission and realization of hereditary information. A gene is a specific section of a DNA molecule, in the structure of which the structure of a certain polypeptide (protein) is encoded. Probably, many DNA regions do not encode proteins, but perform regulatory functions. In any case, in the structure of the human genome, only about 2% of DNA are sequences on the basis of which messenger RNA is synthesized (transcription process), which then determines the amino acid sequence during protein synthesis (translation process). It is currently believed that there are about 30,000 genes in the human genome.

Genes are located on chromosomes, which are located in the nuclei of cells and are giant DNA molecules.

Chromosomal theory of heredity was formulated in 1902 by Setton and Boveri. According to this theory, chromosomes are carriers of genetic information that determines the hereditary properties of an organism. In humans, each cell has 46 chromosomes, divided into 23 pairs. Chromosomes that form a pair are called homologous.

Sex cells (gametes) are formed using a special type of division - meiosis. As a result of meiosis, only one homologous chromosome from each pair remains in each germ cell, i.e. 23 chromosomes. Such a single set of chromosomes is called haploid. At fertilization, when the male and female sex cells merge and a zygote is formed, the double set, which is called diploid, is restored. In the zygote of the organism that develops from it, one chromosome from each nara is received from the paternal organism, the other from the maternal one.

A genotype is a set of genes received by an organism from its parents.

Another phenomenon that genetics studies is variability. Variability is understood as the ability of organisms to acquire new features - differences within a species. There are two types of change:
- hereditary;
- modification (non-hereditary).

hereditary variability- this is a form of variability caused by changes in the genotype, which can be associated with mutational or combinative variability.

mutational variability.
Genes undergo changes from time to time, which are called mutations. These changes are random and appear spontaneously. The causes of mutations can be very diverse. Available whole line factors that increase the risk of mutation. It may be the effect of certain chemical substances radiation, temperature, etc. Mutations can be caused by these means, but the random nature of their occurrence remains, and it is impossible to predict the appearance of a particular mutation.

The resulting mutations are transmitted to descendants, that is, they determine hereditary variability, which is associated with where the mutation occurred. If a mutation occurs in a germ cell, then it has the opportunity to be transmitted to descendants, i.e. be inherited. If the mutation occurred in a somatic cell, then it is transmitted only to those of them that arise from this somatic cell. Such mutations are called somatic, they are not inherited.

There are several main types of mutations.
- Gene mutations, in which changes occur at the level of individual genes, i.e. sections of the DNA molecule. This can be a waste of nucleotides, the replacement of one base with another, a rearrangement of nucleotides, or the addition of new ones.
- Chromosomal mutations associated with a violation of the structure of chromosomes lead to serious changes that can be detected using a microscope. Such mutations include loss of chromosome sections (deletions), addition of sections, rotation of a chromosome section by 180°, and the appearance of repeats.
- Genomic mutations are caused by a change in the number of chromosomes. Extra homologous chromosomes may appear: in the chromosome set, in place of two homologous chromosomes, there are three trisomy. In the case of monosomy, there is a loss of one chromosome from a pair. With polyploidy, a multiple increase in the genome occurs. Another variant of genomic mutation is haploidy, in which only one chromosome from each pair remains.

The frequency of mutations is affected, as already mentioned, by a variety of factors. When a number of genomic mutations occur great importance has, in particular, the age of the mother.

Combination variability.
This type of variability is determined by the nature of the sexual process. With combinative variability, new genotypes arise due to new combinations of genes. This type of variability is manifested already at the stage of formation of germ cells. As already mentioned, each sex cell (gamete) contains only one homologous chromosome from each pair. Chromosomes enter the gamete randomly, so the germ cells of one person can differ quite a lot in the set of genes in the chromosomes. An even more important stage for the emergence of combinative variability is fertilization, after which 50% of the genes of the newly emerged organism are inherited from one parent, and 50% from the other.

Modification variability is not associated with changes in the genotype, but is caused by the influence of the environment on the developing organism.

Availability modification variability very important for understanding the essence of inheritance. Traits are not inherited. You can take organisms with exactly the same genotype, for example, grow cuttings from the same plant, but place them in different conditions (light, humidity, mineral nutrition) and get quite different plants with different traits (growth, yield, leaf shape). and so on.). To describe the actually formed signs of an organism, the concept of "phenotype" is used.

The phenotype is the whole complex of actually occurring signs of an organism, which is formed as a result of the interaction of the genotype and environmental influences during the development of the organism. Thus, the essence of inheritance lies not in the inheritance of a trait, but in the ability of the genotype, as a result of interaction with developmental conditions, to give a certain phenotype.

Since modification variability is not associated with changes in the genotype, modifications are not inherited. Usually this position is for some reason difficult to accept. It seems that if, say, parents train for several generations in lifting weights and have developed muscles, then these properties must be passed on to children. Meanwhile, this is a typical modification, and training is the influence of the environment that influenced the development of the trait. No changes in the genotype occur during modification, and the traits acquired as a result of modification are not inherited. Darwin called this kind of variation - non-hereditary.

To characterize the limits of modification variability, the concept of the reaction norm is used. Some traits in a person cannot be changed due to environmental influences, such as blood type, gender, eye color. Others, on the contrary, are very sensitive to the effects of the environment. For example, as a result of prolonged exposure to the sun, the skin color becomes darker, and the hair lightens. The weight of a person is strongly influenced by the characteristics of nutrition, illness, the presence of bad habits, stress, lifestyle.

Environmental influences can lead not only to quantitative, but also to qualitative changes in the phenotype. In some species of primrose, at low air temperatures (15-20 C), red flowers appear, but if the plants are placed in a humid environment with a temperature of 30 ° C, then white flowers form.

moreover, although the reaction rate characterizes a non-hereditary form of variability (modification variability), it is also determined by the genotype. This provision is very important: the reaction rate depends on the genotype. The same influence of the environment on the genotype can lead to a strong change in one of its traits and not affect the other in any way.

Heredity and variability are among the determining factors in the evolution of the organic world.

Heredity- this is the property of living organisms to preserve and transmit to offspring the features of their structure and development. Due to heredity from generation to generation, the characteristics of a species, variety, breed, strain are preserved. Communication between generations is carried out during reproduction through haploid or diploid cells (see sections "Botany" and "Zoology").

Of the cell organelles, the leading role in heredity belongs to chromosomes capable of self-duplication and formation with the help of genes of the entire complex of traits characteristic of the species (see the chapter "Cell"). The cells of every organism contain tens of thousands of genes. Their totality, characteristic of an individual of a species, is called the genotype.

Variability is the opposite of heredity, but is inextricably linked with it. It is expressed in the ability of organisms to change. Due to the variability of individual individuals, the population is heterogeneous. Darwin distinguished two main types of variability.

Non-hereditary variability(see about modifications in the chapter "Fundamentals of Genetics and Selection") occurs in the process of individual development of organisms under the influence of specific environmental conditions that cause similar changes in all individuals of the same species, therefore Darwin called this variability definite. However, the degree of such changes in individual individuals may be different. For example, grass frogs low temperatures cause a dark color, but its intensity is different in different individuals. Darwin considered modifications to be non-essential to evolution, as they are generally not inherited.

hereditary variability(see about mutations in the chapter "Fundamentals of Genetics and Selection") is associated with a change in the genotype of an individual, so the resulting changes are inherited. In nature, mutations appear in single individuals under the influence of random external and internal factors. Their nature is difficult to predict, so Darwin this variability. named uncertain. Mutations can be minor or major and affect a variety of traits and properties. For example, in Drosophila, under the influence of X-rays, wings, bristles, eye and body color, fertility, etc. change. Mutations can be beneficial, harmful, or indifferent to the organism.

The hereditary variation is combinative variability. It occurs during free crossings in populations or during artificial hybridization. As a result, individuals are born with new combinations of traits and properties that were absent from the parents (see about dihybrid crossing, neoplasms during crossing, chromosome crossing in the chapter "Fundamentals of Genetics and Selection"). Relative variability also hereditary; it is expressed in the fact that a change in one organ causes dependent changes in others (see the chapter "Fundamentals of Genetics and Selection" for the multiple action of a gene). For example, peas with purple flowers always have the same shade of petioles and leaf veins. In wading birds, long limbs and a neck are always accompanied by a long beak and tongue. Darwin considered hereditary variability to be especially important for evolution, since it serves as material for natural and artificial selection in the formation of new populations, species, varieties, breeds and strains.

The textbook complies with the Federal State Educational Standard for Secondary (Complete) General Education, is recommended by the Ministry of Education and Science of the Russian Federation and is included in the Federal List of Textbooks.

The textbook is addressed to students in grade 10 and is designed to teach the subject 1 or 2 hours per week.

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Remember!

Give examples of features that change under the influence of the external environment.

What are mutations?

Variability- one of the most important properties of living things, the ability of living organisms to acquire differences from individuals of both other species and their own species.

There are two types of variability: non-hereditary(phenotypic, or modification) and hereditary(genotypic).

Non-hereditary (modification) variability. This type of variability is the process of the emergence of new traits under the influence of environmental factors that do not affect the genotype. Consequently, the modifications of signs that arise in this case - modifications - are not inherited (Fig. 93). Two identical (monozygous) twins, having exactly the same genotypes, but by the will of fate grown up in different conditions, can be very different from each other. A classic example proving the influence of the external environment on the development of traits is the arrowhead. This plant develops three types of leaves, depending on the growing conditions - in the air, in the water column or on its surface.

Rice. 93. Oak leaves grown in bright light (A) and in a shaded place (B)

Rice. 94. Changing the color of the coat of the Himalayan rabbit under the influence of various temperatures

Under the influence of temperature environment the color of the coat of the Himalayan rabbit changes. The embryo, developing in the womb, is in conditions of elevated temperature, which destroys the enzyme necessary for pigment synthesis, so rabbits are born completely white. Shortly after birth, certain protruding parts of the body (nose, tips of the ears and tail) begin to darken, because there the temperature is lower than in other places, and the enzyme is not destroyed. If you pluck out an area of ​​white wool and cool the skin, black wool will grow in this place (Fig. 94).

Under similar environmental conditions in genetically close organisms, modification variability has group character, for example, in summer period In most people, under the influence of UV rays, a protective pigment, melanin, is deposited in the skin, people sunbathe.

In the same species of organisms, under the influence of environmental conditions, variability various signs can be completely different. For example, in cattle, milk yield, weight, and fertility are very dependent on the conditions of feeding and maintenance, and, for example, the fat content of milk under the influence of external conditions changes very little. Manifestations of modification variability for each trait are limited by their reaction rate. reaction rate- these are the limits in which a change in a trait is possible in a given genotype. In contrast to the modification variability itself, the reaction rate is inherited, and its limits are different for different traits and for individual individuals. The narrowest reaction rate is typical for signs that provide vital important qualities organism.

Due to the fact that most modifications have an adaptive value, they contribute to adaptation - the adaptation of the organism within the limits of the norm of reaction to existence in changing conditions.

Hereditary (genotypic) variability. This type of variability is associated with changes in the genotype, and the traits acquired as a result of this are inherited by the next generations. There are two forms of genotypic variability: combinative and mutational.

Combination variability consists in the appearance of new traits as a result of the formation of other combinations of parental genes in the genotypes of offspring. This type of variability is based on the independent divergence of homologous chromosomes in the first meiotic division, chance meeting gametes in the same parental pair during fertilization and random selection of parental pairs. It also leads to the recombination of genetic material and increases the variability of the exchange of sections of homologous chromosomes, which occurs in the first prophase of meiosis. Thus, in the process of combinative variability, the structure of genes and chromosomes does not change, but new combinations of alleles lead to the formation of new genotypes and, as a result, to the appearance of offspring with new phenotypes.

Mutational variability It is expressed in the appearance of new qualities of the organism as a result of the formation of mutations. The term "mutation" was first introduced in 1901 by the Dutch botanist Hugo de Vries. According to modern ideas mutations- these are sudden natural or artificially induced inherited changes in the genetic material, leading to a change in certain phenotypic characteristics and properties of the organism. Mutations are undirected, that is, random, in nature and are the most important source of hereditary changes, without which the evolution of organisms is impossible. At the end of the XVIII century. in America, a sheep with shortened limbs was born, which gave rise to a new Ancon breed (Fig. 95). in Sweden at the beginning of the 20th century. a mink with platinum fur was born on a fur farm. The huge variety of traits in dogs and cats is the result of mutational variation. Mutations occur abruptly, like new ones qualitative changes: awnless wheat was formed from awned wheat, short wings and striped eyes appeared in Drosophila, white, brown, black color appeared in rabbits from the natural color of agouti as a result of mutations.

According to the place of origin, somatic and generative mutations are distinguished. Somatic mutations arise in the cells of the body and are not transmitted through sexual reproduction to the next generations. Examples of such mutations are age spots and skin warts. generative mutations appear in germ cells and are inherited.

Rice. 95. Ancona sheep

According to the level of change in the genetic material, gene, chromosomal and genomic mutations are distinguished. Gene mutations cause changes in individual genes, disrupting the order of nucleotides in the DNA chain, which leads to the synthesis of an altered protein.

Chromosomal mutations affect a significant portion of the chromosome, disrupting the functioning of many genes at once. A separate fragment of the chromosome can double or be lost, which causes serious disturbances in the functioning of the body, up to the death of the embryo in the early stages of development.

Genomic mutations lead to a change in the number of chromosomes as a result of violations of the divergence of chromosomes in the divisions of meiosis. The absence of a chromosome or the presence of an extra one leads to adverse consequences. Most famous example Down syndrome is a genomic mutation, a developmental disorder that occurs when an extra 21st chromosome appears. Such people have total number chromosomes is 47.

In protozoa and in plants, an increase in the number of chromosomes, a multiple of the haploid set, is often observed. This change in the chromosome set is called polyploidy(Fig. 96). The emergence of polyploids is associated, in particular, with the nondisjunction of homologous chromosomes during meiosis, as a result of which not haploid, but diploid gametes can form in diploid organisms.

mutagenic factors. The ability to mutate is one of the properties of genes, so mutations can occur in all organisms. Some mutations are incompatible with life, and the embryo that received them dies in the womb, while others cause persistent changes in traits that are significant to varying degrees for the life of the individual. Under normal conditions, the mutation rate of an individual gene is extremely low (10–5), but there are environmental factors that significantly increase this value, causing irreversible damage to the structure of genes and chromosomes. Factors whose impact on living organisms leads to an increase in the frequency of mutations are called mutagenic factors or mutagens.

Rice. 96. Polyploidy. Chrysanthemum flowers: A - diploid form (2 n); B - polyploid form

All mutagenic factors can be divided into three groups.

Physical mutagens are all types of ionizing radiation (?-rays, x-rays), ultraviolet radiation, high and low temperatures.

Chemical mutagens are analogues. nucleic acids, peroxides, salts of heavy metals (lead, mercury), nitrous acid and some other substances. Many of these compounds cause disturbances in DNA replication. Substances used in agriculture for pest and weed control (pesticides and herbicides), industrial waste, certain food colors and preservatives, some drugs, tobacco smoke components.

Special laboratories and institutes have been set up in Russia and other countries of the world to test all newly synthesized chemical compounds for mutagenicity.

To the group biological mutagens include foreign DNA and viruses that, embedding in the host's DNA, disrupt the work of genes.

Review questions and assignments

1. What kinds of variability do you know?

2. What is a reaction rate?

3. Explain why phenotypic variability is not inherited.

4. What are mutations? Describe the main properties of mutations.

5. Give a classification of mutations according to the level of changes in the hereditary material.

6. Name the main groups of mutagenic factors. Give examples of mutagens that belong to each group. Assess if there are mutagenic factors in your environment. What group of mutagens do they belong to?

Think! Execute!

1. In your opinion, can environmental factors affect the development of an organism carrying a lethal mutation?

2. Can combinative variability manifest itself in the absence of the sexual process?

3. Discuss in class what are the ways to reduce human exposure to mutagenic factors in today's world.

4. Can you give examples of modifications that are not adaptive in nature?

5. Explain to someone unfamiliar with biology how mutations differ from modifications.

6. Perform the study: "The study of modification variability in students (for example, body temperature and pulse rate, periodically measured for 3 days)".

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

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Variability is a process that reflects the relationship of an organism with the environment.

From a genetic point of view, variability is the result of the reaction of the genotype in the process of individual development of the organism to environmental conditions.

The variability of organisms is one of the main factors of evolution. It serves as a source for artificial and natural selection.

Biologists distinguish between hereditary and non-hereditary variability. Hereditary variability includes such changes in the characteristics of an organism that are determined by the genotype and persist over a number of generations. To non-hereditary variability, which Darwin called definite, and is now called modification, or phenotypic, variability, refer to changes in the characteristics of the organism; not preserved during sexual reproduction.

hereditary variability is a change in the genotype non-hereditary variability- change in the phenotype of the organism.

During individual life An organism under the influence of environmental factors can experience two types of changes: in one case, the functioning, the action of genes in the process of trait formation, changes, in the other, the genotype itself.

We got acquainted with hereditary variability resulting from combinations of genes and their interaction. The combination of genes is carried out on the basis of two processes: 1) independent distribution of chromosomes in meiosis and their random combination during fertilization; 2) chromosome crossing and gene recombination. Hereditary variability due to the combination and recombination of genes is commonly called combinative variability. With this type of variability, the genes themselves do not change, their combination and the nature of interaction in the genotype system change. However, this type of hereditary variability should be considered as a secondary phenomenon, and the mutational change in the gene should be considered primary.

The source for natural selection is hereditary changes - both mutations of genes and their recombination.

Modification variability plays a limited role in organic evolution. So, if you take vegetative shoots from the same plant, such as strawberries, and grow them in various conditions humidity, temperature, illumination, on different soils, then despite the same genotype, they will be different. The action of various extreme factors can cause even greater differences among them. However, seeds collected from such plants and sown under the same conditions will give the same type of offspring, if not in the first, then in subsequent generations. Changes in the signs of the organism, caused by the action of environmental factors in ontogenesis, disappear with the death of the organism.

At the same time, the capacity for such changes, limited by the limits of the normal reaction of the organism's genotype, has an important evolutionary significance. As shown by A.P. Vladimirsky in the 1920s, V.S. Kirpichnikov and I.I. Shmalgauzen in the 1930s, in the case when modification changes in the adaptive value occur with environmental factors constantly acting in a number of generations, which able to cause mutations that determine the same changes, one may get the impression of hereditary fixation of modifications.

Mutational changes are necessarily associated with the reorganization of the reproducing structures of germ and somatic cells. Fundamental difference mutations from modifications is reduced to the fact that mutations can be accurately reproduced in a long series of cell generations, regardless of the environmental conditions in which ontogenesis takes place. This is explained by the fact that the occurrence of mutations is associated with a change in the unique structures of the cell - the chromosome.

On the question of the role of variability in evolution, there was a long discussion in biology in connection with the problem of inheritance of the so-called acquired traits, put forward by J. Lamarck in 1809, partly accepted by Charles Darwin and still supported by a number of biologists. But the vast majority of scientists considered the very formulation of this problem unscientific. At the same time, it must be said that the idea that hereditary changes in the body arise adequately to the action of an environmental factor is completely absurd. Mutations occur in a variety of ways; they cannot be adaptive for the organism itself, since they arise in single cells

And their action is realized only in offspring. Not the factor that caused the mutation, but only selection evaluates the adaptive knowledge of the mutation. Since the direction and pace of evolution are determined by natural selection, and the latter is controlled by many factors of the internal and external environment, a false idea is created about the initial adequate expediency of hereditary variability.

Selection on the basis of single mutations "constructs" systems of genotypes that meet the requirements of those permanent conditions in which the species exists.

The term " mutation"was first proposed by G. de Vries in his classic work" Mutation Theory "(1901-1903). Mutation he called the phenomenon of a spasmodic, discontinuous change in a hereditary trait. The main provisions of the theory of de Vries have not lost their significance so far, and therefore they should be given here:

1. mutation occurs suddenly, without any transitions;
2. the new forms are completely constant, that is, they are stable;
3. Mutations, unlike non-hereditary changes (fluctuations), do not form continuous series, they are not grouped around an average type (mode). Mutations are qualitative changes;
4. mutations go in different directions, they can be both beneficial and harmful;
5. mutation detection depends on the number of individuals analyzed for mutation detection;
6. the same mutations can occur repeatedly.

However, G. de Vries made a fundamental mistake by opposing the theory of mutations to the theory of natural selection. He incorrectly believed that mutations could immediately give rise to new species adapted to the external environment, without the participation of selection. In fact, mutations are only a source of hereditary changes that serve as material for selection. As we will see later, gene mutation is only evaluated by selection in the genotype system. The error of G. de Vries is connected, in part, with the fact that the mutations he studied in evening primrose (Oenothera Lamarciana) subsequently turned out to be the result of splitting a complex hybrid.

But one cannot but admire the scientific foresight that H. de Vries made regarding the formulation of the main provisions of the mutation theory and its significance for selection. Back in 1901, he wrote: “...mutation, mutation itself, should become the object of study. And if we ever succeed in elucidating the laws of mutation, then not only will our view of the mutual relationship of living organisms become much deeper, but we also dare to hope that the possibility of mastering mutability should open up as well as the breeder dominates variability, variability. Of course, we will come to this gradually, mastering individual mutations, and this will also bring many benefits to agricultural and horticultural practice. Much that now seems unattainable will be within our power, if only we can learn the laws on which the mutation of species is based. Obviously, here we are waiting for an boundless field of persistent work. high value for both science and practice. This is a promising area for dominating mutations.” As we will see later, modern natural science is on the threshold of understanding the mechanism of gene mutation.

The theory of mutations could develop only after the discovery of Mendel's laws and the laws established in the experiments of the Morgan school of gene linkage and their recombination as a result of crossing over. Only since the establishment of the hereditary discreteness of chromosomes, the theory of mutations received a basis for scientific research.

Although at present the question of the nature of the gene has not been completely elucidated, a number of general patterns of gene mutation have nevertheless been firmly established.

Gene mutations occur in all classes and types of animals, higher and lower plants, multicellular and unicellular organisms, bacteria and viruses. Mutational variability as a process of qualitative spasmodic changes is universal for all organic forms.

Purely conventionally, the mutation process is divided into spontaneous and induced. In cases where mutations occur under the influence of ordinary natural environmental factors or as a result of physiological and biochemical changes in the organism itself, they are referred to as spontaneous mutations. Mutations arising under the influence of special influences (ionizing radiation, chemicals, extreme conditions etc.), are called induced. There are no fundamental differences between spontaneous and induced mutations, but the study of the latter leads biologists to master hereditary variability and unravel the mystery of the gene.